Long training sequence method and device for wireless communications

ABSTRACT

A method and device for transmitting a frame of a wireless communication begins by generating a preamble of the frame that includes a short training sequence and at least one long training sequence. The at least one long training sequence includes non-zero energy on each of a plurality of subcarriers except a DC subcarrier. The at least one long training sequence corresponds to the number of antennas and applicable wireless communication standards. A matrix is defined to represent the at least one long training sequence. The preamble is compatible with legacy and current standards. A channel is defined with a set of sub carriers to transmit the frame.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is claiming priority under 35 USC § 119 to thefollowing co-pending patent applications: U.S. Provisional PatentApplication Ser. No. 60/544,605, filed on Feb. 13, 2004; Ser. No.60/545,854, filed on Feb. 19, 2004; and U.S. Provisional PatentApplication Ser. No. 60/568,914, filed on May 7, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to wireless communication systems andmore particularly to supporting multiple wireless communicationprotocols within a wireless local area network using a long trainingsequence.

2. Description of Related Art

Wireless and wire lined communications are supported by current andlegacy devices within existing networks and systems. Communicationsystems may include from national or international cellular telephonesystems to, the Internet, and point-to-point in-home wireless networks.A communication system is constructed, and may operate, in accordancewith one or more communication standards or protocols. Wirelesscommunication systems may operate in accordance with one or morestandards including, but not limited to, IEEE 802.11, Bluetooth,advanced mobile phone services (AMPS), digital AMPS, global system formobile communications (GSM), code division multiple access (CDMA), localmulti-point distribution systems (LMDS), multi-channel-multi-pointdistribution systems (MMDS), and the like.

The IEEE 802.11 specification has evolved from IEEE 802.11 to IEEE802.11b to IEEE 802.11a and to IEEE 802.11g. Wireless communicationdevices that are compliant with IEEE 802.11b (standard 11b) may exist inthe same wireless local area network (WLAN) as IEEE 802.11g (standard11g) compliant wireless communication devices. Further, IEEE 802.11a(standard 11a) compliant wireless communication devices may reside inthe same WLAN as standard 11g compliant wireless communication devices.

The different standards may operate within different frequency ranges,such as 5 to 6 gigahertz (GHz) or 2.4 GHz. For example, standard 11a mayoperate within the higher frequency range. One aspect of standard 11a isthat portions of the spectrum between 5 to 6 GHz are allocated to achannel. The channel may be 20 megahertz (MHz) wide within the frequencyband. Standard 11a also may use orthogonal frequency divisionmultiplexing (OFDM). OFDM may be implemented over sub-carriers thatrepresent lines, or values, within the frequency domain of the 20 MHzchannels. The signal may be transmitted over many different sub-carrierswithin the channel. The sub-carriers are orthogonal to each other sothat information may be extracted off each sub-carrier about the signal.

A communication system also may include legacy wireless devices. Legacydevices are those devices compliant with an earlier version of thestandard, but reside in the same WLAN as devices compliant with a laterversion of the standard. A mechanism may be employed to ensure thatlegacy devices know when the newer version devices are utilized in thewireless channel to avoid a collision.

Thus, newer devices or components using current standards should havebackward compatibility with already installed equipment within anetwork. These devices and components should be adaptable to legacystandards and current standards when transmitting information within thenetwork. Legacy devices or components may be kept off the air or out ofthe network so as not to interfere or collide with information that theyare not familiar with. For example, if the legacy device receives asignal or information supported by a newer standard, then the deviceshould forward the information or signal to the appropriate destinationwithout modifying or terminating the signal or information. Further, thereceived signal information may not react to the legacy device as if thelegacy device is a device compatible with the newer standard.

For example, backward compatibility with legacy devices may be enabledexclusively at either the physical (PITY) layer or the Media-SpecificAccess Control (MAC) layer. At the PHY layer, backward compatibility isachieved by re-using the PHY preamble from a previous standard. Legacydevices may decode the preamble portion of all signals, which providessufficient information for determining that the wireless channel is inuse for a specific period of time, and avoid interference even thoughthe legacy devices cannot fully demodulate or decode the transmittedframe(s).

At the MAC layer, backward compatibility with legacy devices may beenabled by forcing devices that are compliant with a newer version ofthe standard to transmit special frames using modes or data rates thatare employed by legacy devices. These special frames contain informationthat sets the network allocation vector (NAV) of legacy devices suchthat these devices know when the wireless channel is in use by newerstations.

Mechanisms for backward compatibility may suffer from a performance lossrelative to that which can be achieved without backward compatibilityand are used independently of each other. Further, in standard 11a and11g transmitters, only 52 subcarriers (−26 . . . −1 and +1 . . . +26)are filled with non-zero values even though an IFFT (inverse fastFourier transform) of length 64 may be used. As such, sharpfrequency-domain transitions may occur between zero subcarriers andnon-zero subcarriers, which results in a time-domain ringing. Thisadversely affects a receiver's ability to detect a valid preambletransmission and requires the receiver to perform a channel estimateusing the full fast Fourier transform (FFT) size.

SUMMARY OF THE INVENTION

A method is disclosed for transmitting a frame for wirelesscommunications. The method includes determining at least one firsttraining sequence according to a number of antennas for transmitting aframe. The method also includes generating a preamble for the frame. Thepreamble includes a second training sequence and the at least one firsttraining sequence. The method also includes transmitting the preamblewith the frame using the number of antennas.

A method also is disclosed for transmitting data in a wireless system.The method includes defining a matrix for training of at least oneantenna. The method also includes determining at least one firsttraining sequence according to the matrix and a standard for wirelesscommunication. The method also includes generating a preamble includingthe at least one first training sequence and a second training sequence.The preamble is compatible with the standard.

A device for wireless communication also is disclosed. The deviceincludes a transceiver to transmit a signal. The signal includes a framehaving a preamble. The device also includes a subset of subcarriers totransmit the signal within a channel. The device also includesprocessing means to produce at least one data stream for the frame andto generate the preamble. The preamble includes an expanded at least onefirst training sequence and a second training sequence.

A method also is disclosed for generating a frame for wirelesscommunications. The method includes generating a preamble having aplurality of first training sequences and a second training sequence.The preamble is defined by a wireless communication standard. The methodalso includes determining the plurality of first training sequencesaccording to the wireless communication standard. The method alsoincludes defining a channel according to the wireless communicationstandard. The method also includes stimulating a set of sub carriers forthe channel with the plurality of first training sequences.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a wireless communication system inaccordance with the present invention;

FIG. 2 illustrates a block diagram of a wireless communication device inaccordance with the present invention;

FIG. 3 illustrates a block diagram of an RF transmitter in accordancewith the present invention;

FIG. 4 illustrates a block diagram of a processor to generate anexpanded long training in accordance with the present invention;

FIG. 5 illustrates frames for wireless communication between twowireless devices in accordance with the present invention;

FIG. 6 illustrates frames for wireless communication between twowireless devices in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 depicts a communications system 10 according to the presentinvention. Communications system 10 may include stations 14, 16 and 18.Stations 14, 16 and 18 may include wireless communication devices, suchas cellular or wireless devices, digital devices, laptop or desktopcomputers, personal digital systems, and the like. Stations 14, 16 and18 may be coupled to network 12, which transmits data information withincommunications system 10. Additional stations and applicable devices orcomponents also may be coupled to network 12 within communicationssystem 10.

Communications system 10 may forward data or information in the form ofsignals, either analog or digital. Wireless devices within theindividual stations may register with the station and receive servicesor communications within communications system 10. Wireless devices mayexchange data or information via an allocated channel. Network 12 mayset up local area networks, such as local area network 12 a (LAN), widearea networks, such as wide area network 12 c (WAN), wireless local areanetworks, such as wireless local area network 12 b (WLAN), ad-hocnetworks and the like.

Communications system 10 may operate according to the IEEE 802.11n(standard 11n) protocol for wireless communications. Alternatively,communications system 10 may operate under a variety of standards orprotocols, such as standard 11a, and standard 11n, and include legacydevices or components. For example, certain components may comply withstandard 11a while newer components may comply with standard 11n.Standard 11n may occupy the 5-6 GHz band, or, alternatively standard 11nmay occupy the 2.4 GHz band. Standard 11n also may be considered anextension of standard 11a. Standard 11n devices and components mayoperate with a bandwidth of 100 MHz. The devices and components may knowthe physical layers rate for standard 11n devices and components ofcommunication system 10 may be greater than those of previous standards.Further, the bandwidth for channels of standard 11n may be 20 MHz or 40MHz. Thus, standard 11n may implement wider channel bands than previousstandards. For example, instead of 20 MHz bands, standard 11n may puttwo bands together as a 40 MHz band and may send twice as much data.Moreover, information may be filled in the gap between the two 20 MHzbands and their falloffs. By filling in these gaps, data or informationsent according to standard 11n might over be twice as much as that sentaccording to legacy standards.

Standard 11n may be applicable to different configurations ofcommunication system 10. For example, antennas may be used in thewireless devices and components in communications system 10. In order tooperate multiple transmitters, communications system 10 may havemultiple receivers so that several different signals may be transmittedwithin communications system 10. The number of receivers may bedependent upon the number of streams of data or the number oftransmitters. For example, the number of receivers within communications10, or any device or component thereof, may be equal to or greater thanthe number of data streams.

Therefore, communications system 10 may include a multiple input,multiple output (MIMO) structure. MIMO structures may be implemented incommunications system 10 to improve robustness of wirelesscommunications. To better improve robustness, communications system 10also may set the number of data streams to be less than the number oftransmitters in a wireless device. Depending on the number oftransmitters within the device, the effectiveness of the transmissionand reception of signals may be determined.

Various parameters may be taken into account regarding transmissionchannels under standard 11n, as well as previous standards. For example,the transmission channel may have certain shapes or wave forms. Datarates of the signals may be derived from the expanded bandwidth in thenumber of transmission. On the receiver side, channel estimation may beachieved by using training within the preamble of a signal. On thetransmitter side, channel sounding may be used to determine what thetransmitter is supposed to send. Channel estimation may relate to whatsort of signal is sent, what the signal looks like, and how the signalmay be received. For example, standard 11n may implement long trainingsequences to provide channel estimation and sounding.

Communications system 10 may resolve the issue of taking standard 11asignals and having the signals operate within a MIMO system usingmultiple antennas. For example, communications system 10 may determinehow the standard 11a signals will work within the wider bandwidth ofstandard 11n. Thus, communications system 10 may increase theprobability of reception of signals transmitting large amounts of data.One factor may be the presumption that all of the devices and componentswithin communications system 10 may receive all transmitted signals, nomatter what format or standard is used.

FIG. 2 depicts a block diagram illustrating a wireless communicationdevice according to the present invention. The wireless device mayinclude host device 18 and an associated radio 60. For cellulartelephone hosts, radio 60 is a built-in component. For personal digitalassistants hosts, laptop hosts, or personal computer hosts, radio 60 maybe built-in or an externally coupled component.

Radio 60 may include host interface 62, baseband processing module 63,memory 65, plurality of radio frequency (RF) transmitters 67, 69, and71, transmit/receive (T/R) module 73, plurality of antennas 81, 83, and85, plurality of RF receivers 75, 77, and 79, and local oscillationmodule 99. Baseband processing module 63, in combination withoperational instructions stored in memory 65, may execute digitalreceiver functions and digital transmitter functions. Basebandprocessing modules 63 may be implemented using one or more processingdevices. Memory 65 may be a single memory device or a plurality ofmemory devices. When processing module 63 implements one or more of itsfunctions via a state machine, analog circuitry, digital circuitry, orlogic circuitry, memory 65 may store the corresponding operationalinstructions may be embedded with the circuitry comprising the statemachine, analog circuitry, digital circuitry, or logic circuitry.

In operation, radio 60 receives outbound data 87 from host device 18 viahost interface 62. Baseband processing module 63 receives outbound data87 and, based on a mode selection signal 101, produces one or moreoutbound symbol streams 89. Mode selection signal 101 may indicate aparticular mode. For example, mode selection signal 101, may indicate afrequency band, a channel bandwidth of 20 or 22 MHz and a maximum bitrate of 54 megabits-per-second. Mode selection signal 101 also mayindicate a particular rate ranging from 1 megabit-per-second to 54megabits-per-second. In addition, mode selection signal 101 may indicatea particular type of modulation, which includes, but is not limited to,Barker Code Modulation, BPSK, QPSK, CCK, 16 QAM and/or 64 QAM. A coderate is supplied as well as number of coded bits per subcarrier (NBPSC),coded bits per OFDM symbol (NCBPS), data bits per OFDM symbol (NDBPS),error vector magnitude in decibels (EVM), sensitivity that indicates themaximum receive power required to obtain a target packet error rate(e.g., 10% for standard 11a), adjacent channel rejection (ACR), and analternate adjacent channel rejection (AACR).

Baseband processing module 63, based on mode selection signal 101 mayproduce one or more outbound symbol streams 89 from output data 88. Forexample, if mode selection signal 101 indicates that a single transmitantenna is being utilized for the particular mode that has beenselected, baseband processing module 63 may produce a single outboundsymbol stream 89. Alternatively, if mode selection signal 101 indicates2, 3 or 4 antennas, baseband processing module 63 may produce 2, 3 or 4outbound symbol streams 89 corresponding to the number of antennas fromoutput data 88.

Depending on the number of outbound streams 89 produced by basebandmodule 63, a corresponding number of RF transmitters 67, 69, and 71 maybe enabled to convert outbound symbol streams 89 into outbound RFsignals 91. T/R module 73 receives outbound RF signals 91 and provideseach outbound RF signal to a corresponding antenna 81, 83 and 85.

When radio 60 is in the receive mode, T/R module 73 receives one or moreinbound RF signals via antennas 81, 83, and 85. T/R module 73 providesinbound RF signals 93 to one or more RF receivers 75, 77, and 79.Inbound RF signals 93 are converted into a corresponding number ofinbound symbol streams 96. The number of inbound symbol streams 95 maycorrespond to the particular mode in which the data was received.Baseband processing module 63 receives inbound symbol streams 89 andconverts them into inbound data 97, which is provided to host device 18via host interface 62.

FIG. 3 depicts a block diagram of an RF transmitter 200 according to thepresent invention. RF transmitter 200 may include a filter 202, adigital-to-analog conversion module 204, a filter 206, and anup-conversion module 208. RF transmitter 200 also may include a poweramplifier 210 and an RF filter 212. Filter 202 may include a digitalfilter that receives one of outbound signal streams 222. Filter 202 maydigitally filter an outbound signal and may up-sample the rate ofoutbound signal streams 222 to a desired rate to produce filtered signalstreams 224. Digital-to-analog conversion (D/A) module 204 may convertthe filtered signal streams 224 into analog signals 226. Analog signals226 may include an in-phase component and a quadrature component.

Filter 206 may be an analog filter that filters analog signals 226 toproduce filtered analog signals 228. Up-conversion module 208 mayinclude a pair of mixers and a filter. Up-conversion module 208 may mixfiltered analog signals 228 with a local oscillation signal that isproduced by local oscillation module 214. Up-conversion module 208 mayproduce high frequency signals 230. The frequency of high frequencysignals 230 may correspond to the frequency outbound RF signals 234.Power amplifier 210 may amplify high frequency signals 230 to produceamplified high frequency signals 232. RF filter 212 may be a highfrequency band-pass filter that filters amplified high frequency signals232 to produce output RF signals 234.

Transmitter 200 may be configured to generate or create signals fromdigital data or packets that are supported by standard 11n. Further,transmitter 200 may generate or create signals to perform channelestimation or channel sounding. Transmitter 200 also may be coupled tomultiple antennas 380. Antennas 380 may support the MIMO wirelesscommunication used in standard 11n. Alternatively, transmitter 200 maybe coupled to a single input single output configuration that complieswith legacy standards, such as standard 11a or 11g.

Transmitter 200 may transmit to multiple receivers within the network orcommunication system. Further, additional transmitters may be coupled toantennas 380 to transmit signals. Transmitter 200 also may beimplemented in a device or component within a communication system. Forexample, transmitter 200 may be implemented in a wireless device thatexchanges data or information within a wireless local area network.

FIG. 4 depicts a block diagram of a processor 400 configured to generatean expanded long training sequence according to the present invention.Processor 400 may include a symbol mapper 402, an inverse fast Fouriertransform (IFFT) module 404, a serial to parallel module 406, a digitaltransmit filter or time domain window module 408, and digital to analogconverters (D/A) 410 and 412.

For an expanded long training sequence, symbol mapper 402 may generatesymbols from coded bits 414 for each of the 64 subcarriers of anorthogonal frequency division multiplexings (OFDM) sequence. IFFT module404 may convert the subcarriers of an applicable channel from thefrequency domain to the time domain. Serial to parallel module 406 mayconvert the serial time domain signals into parallel time domain signalsthat are subsequently filtered and converted to analog signals via D/Aconverters 410 and 412.

Transmitter 400 may generate or create frame 440. Frame 440 may beencoded and placed in a signal generated by transmitter 400. Frame 440may include preamble 442 and data field 444. Frame 440 may be supportedby standard 11n. Standard 11n may apply to frame based communicationsystems, which systems include transmitter 400.

Preamble 442 may be applicable to standard 11n, standard 11a andstandard 11g protocol network and systems and include a short trainingand a long training sequence. A short training sequence may be about 8microseconds and provides a rough synchronization, identification, acoarse frequency check and auto gain control for frame 440. A longtraining sequence may perform fine frequency acquisition and channelestimation. The long training sequence also may be referred to as thefirst training sequence, while the short training sequence may bereferred to as the second training sequence. Preamble 442 may prefer areceiver that receives frame 440 for proper transmission or reception ofsignals.

Preamble 442 may address backward compatibility between frame 440 andlegacy devices or components already installed within the network.Legacy devices or components may support previous standards orprotocols, such as standard 11a or 11g. In transmitting frames orsignals according to standard 11n, legacy devices, components, stations,and the like may be kept at the network using the physical layer. Thus,preamble 442 may trick standard 11a and standard 11g stations to stayoff the network when a frame, or signal, supported by standard 11n istransmitted. Preamble 442, however, also may be a valid headerrecognizable by the different legacy and current stations.

Preamble 442 may be a physical header. Two types of physical headers mayapply in generating frame 440. A Greenfield header may be used when novalid standard 11a or standard 11g preamble is desired. A Brownfieldheader, or legacy header, may be used when a valid standard 11a orstandard 11g preamble is desired. A Brownfield header may incur a 20microsecond penalty by adding extra long training or additional fieldswithin preamble 442 or data field 444. Legacy devices, however,receiving a Brownfield header know to stay off the network and to notcollide or interfere with frame 440. Multiple sequences may be added topreamble 442 for frames supported by standard 11n.

FIG. 5 depicts frames for wireless communication between two wirelesscommunication devices according to the present invention. The wirelesscommunication devices may be in a proximal region where the protocolthat is used is standard 11n. The wireless communication may be direct,such as from a wireless communication device to a wireless communicationdevice, or indirect, such as from a wireless communication device to anaccess point to a wireless communication device. For example, firstwireless communication device may provide frame 504 to a second wirelesscommunication device using multiple antennas. Frame 504 may include awireless communication set-up information field 506 and a data field508.

Set-up information field 506 may include a short training sequence 510that may be about 8 microseconds long, a 1^(st) supplemental longtraining sequence 512 that may be about 4 microseconds long, which maybe one of a plurality of supplemental long training sequences 514, and asignal field 510 that may be about 4 microseconds long. The number ofsupplemental long training sequences 514 may correspond to the number oftransmit antennas utilized for multiple input multiple output (MIMO)radio communications. One or more of supplemental long trainingsequences 514 may be expanded, as described above.

Data field 508 of frame 504 includes a plurality of data symbols 518,each being about 4 microseconds in duration. Last data symbol 520 alsomay include applicable tail bits and padding bits in addition to datasymbols.

The preamble, which may be referred to as a “Greenfield” header orpreamble, is used when standard 11n devices are present. Alternatively,the preamble may be used with legacy devices (.11,.11a,.11b, and .11g)when MAC level protection, such as RTS/CTS or CTS to self, is employed.MAC level protection may also be used when legacy stations are notpresent to protect very long bursts.

Short training sequence 510 may be the same as one for standard 11a forTX antenna 1. For antennas 2 to N, a cyclic shifted version of the samesequence may be used. The amount of cyclic shift per antenna may becomputed from (Antenna number—1)*800/N in nanoseconds. Thus for antenna1, the shift may be zero. For 2 antennas, the shift may be 0 ns forantenna 1 and 400 ns for antenna 2. For 3 antennas, the shifts may be 0,250, and 500 ns. For 4 antennas, the shifts may be 0, 200, 400, and 600ns. The implementation may include the shifts being rounded to units of50 ns for the inverse of the symbol clock frequency. Shifts may beimplemented in either a forward or backward direction.

Several possible implementations of the supplemental long trainingsequences may exist: (m=1). For example, this implementation may includeone long training sequence. For antenna 1, it may be the same as thestandard 11a long training sequence but about 4 microseconds long,including a 0.8 microsecond guard interval. For antennas 2 to N, it maybe a cyclic shifted version of the same sequence. The amount of cyclicshift per antenna may be computed from (Antenna number—1)*4/N inmicroseconds. Thus for 1 antenna, the shift may be zero. For 2 antennas,the shift may be 0 ns for antenna 1 and 4 us for antenna 2. For 3antennas, the shifts may be 0, 2.65 us, and 5.35 us. For 4 antennas, theshifts may be 0, 2, 4, and 6 microseconds. The shifts may be rounded tounits of 50 ns or the inverse of the symbol clock frequency. Shifts maybe implemented in either a forward or backward direction.

For (m=N), the number of training sequences may be equal to the numberof transmit antennas. This example may differ from the (m=1) examplebecause of less channel estimation error at the receiver, especially fora large numbers of antennas. Thus, it may be scalable. Thus, thefollowing choices of training sequence may exist:

Zero space—sequences (1,1), (2,2), (3,3), . . . up to (N,N) may be thesame as the standard 11a long training sequence. All others (i.e. (1,2),(2,1), etc) may be null so that nothing is transmitted during that timeslot.)

Subchannel null—the set of sub-channels in the training sequences may besub-divided by the number of transmit antennas. Individual subsets maybe activated on each sub-training interval.

Orthogonal sequences for OFDM transmission may be generated bymultiplying the subcarriers of the standard 11a long training sequenceby an m×m orthonormal matrix, which generates a discrete Fouriertransform.

FIG. 6 depicts frames for wireless communication between two wirelesscommunication devices according the present invention. The wirelesscommunication devices may be compliant with standard 11n. Such acommunication may take place within a proximal area that includesstandard 11n compliant devices, standard 11a compliant devices orstandard 11g compliant devices. The wireless communication may be director indirect where a frame 610 includes a legacy filed of set-upinformation 612, remaining set-up information field 614, and the field608 using multiple antennas.

The legacy portion of set-up information 612 includes a short trainingsequence 616, which is about 8 microseconds in duration, a long trainingsequence 618, which is about 8 microseconds in duration, and a signalfield 620, which is about 4 microseconds in duration. Signal field 620may include several bits to indicate the duration of frame 610. As such,standard 11a compliant devices within the proximal area and the standard11g compliant devices within the proximal area may recognize that aframe is being transmitted even though such devices may not be able tointerpret the remaining portion of the frame. Thus, the legacy devicesof standard 11a and standard 11g may avoid interference or avoidcollision with the standard 11n communication based on a properinterpretation of the legacy portion of set-up information 612.

Remaining set-up information 614 may include additional supplementallong training sequences 618 and 620, which are each about 4 microsecondsin duration. Remaining set-up information 614 also may include a highdata signal field 626, which is about 4 microseconds in duration toprovide additional information regarding frame 610. Data field 608 mayinclude data symbols 628, which are about 4 microseconds in duration.Last data symbol 630 may also include tail and padding bits. Thus, thelegacy protection may be provided at the physical layer. One or moresupplemental long training sequences 624 may be expanded, as describedabove.

For example, m may be the number of longer training sequences per frame,N is the number of transmit antennas, the preamble also referred to as“Brownfield” may be when standard 11a or standard 11g legacy devices orcomponents present. Short training sequence 616 and long trainingsequence 618 are the same as standard 11a for TX antenna 1. For antennas2 to N, the following process may exist:

Use a cyclic shifted version of the same sequence. The amount of cyclicshift per antenna may be computed from (Antenna number—1)*800/N innanoseconds for the short training and (Antenna number—1)*4/N inmicroseconds

Another mode is to leave the short training through signal field partstransmitted on antennas 2 to N as null, such that these antennas do nottransmit during this interval. Supplemental long training sequences 624from antenna 1 are not used and nothing is transmitted during this time.

Signal field 620 may follow the same format as standard 11a, except thereserved bit (4) may be set to 1 to indicate a standard 11n frame andsubsequent training for standard 11n receivers. Supplemental longtraining sequences 624 may be defined as follows:

-   -   (m=1) There may be one long supplemental training sequence. It        may be orthogonal to the standard 11a long training sequence.    -   (m=N−1) The number of training sequences may be equal to the        number of transmit antennas. This example is different from the        (m=1) example because it may lead to less channel estimation        error at the receiver, especially for large numbers of antennas.        Thus, it may be scalable.

The following are some possible choices of a training sequence:

-   -   Zero space—For example, sequences (1,1), (2,2), (3,3), . . . up        to (m,m) are the same as the 802.11a long training sequence. All        others (i.e. (1,2), (2,1), etc) may be null such that nothing is        transmitted during that time slot.    -   Subchannel null—For example, the set of sub-channels in the        training sequences may be sub-divided by the number of transmit        antennas. Individual subsets are activated on each sub-training        interval.

Orthogonal sequences for OFDM transmission may be generated bymultiplying the standard 11a long training sequence by an m×morthonormal matrix, which generates a discrete Fourier transform. Forexample, the 4 antenna example may employ the following orthonormalmatrix (Equation 01) to generate the subcarriers for each supplementallong training sequence. $\begin{matrix}{{S_{k} = {\begin{bmatrix}s_{10,k} & s_{11,k} & s_{12,k} \\s_{20,k} & s_{21,k} & s_{22,k} \\s_{30,k} & s_{31,k} & s_{32,k}\end{bmatrix}\quad = \begin{bmatrix}s_{00,k} & {s_{00,k} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot \theta_{k}}} & {s_{00,k} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot \phi_{k}}} \\s_{00,k} & {s_{00,k} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {({\theta_{k} - \frac{4 \cdot \pi}{3}})}}} & {s_{00,k} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {({\phi_{k} - \frac{2 \cdot \pi}{3}})}}} \\s_{00,k} & {s_{00,k} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {({\theta_{k} - \frac{2 \cdot \pi}{3}})}}} & {s_{00,k} \cdot {\mathbb{e}}^{{\mathbb{i}} \cdot {({\phi_{k} - \frac{4 \cdot \pi}{3}})}}}\end{bmatrix}}}\begin{matrix}{\theta_{k} = {\pi \cdot {k/\left( {4 \cdot N_{subcarriers}} \right)}}} \\{\phi_{k} = {\pi \cdot {\left( {k + 4} \right)/\left( {2 \cdot N_{subcarriers}} \right)}}}\end{matrix}} & (01)\end{matrix}$

FIG. 7 depicts frames for wireless communication between two wirelesscommunication devices according the present invention. The wirelesscommunication devices may be standard 11n compliant using multipleantennas. The wireless communication may be direct or indirect within aproximal area that includes standard 11n compliant devices, standard11a, standard 11b, and standard 11g devices. For example, frame 710 mayinclude a legacy portion of set-up information field 712, remainingset-up information field 714 and data field 708. The legacy portion ofset-up information 712 includes an IEEE 802.11 PHY preamble with shorttraining sequence 716, long training sequence 718, signal field 720 anda MAC partitioning frame field 722. MAC field 722 may indicate theparticulars of frame 710 that may be interpreted by legacy devices. Forexample, the legacy protection is provided at the MAC layer. The fieldsmay follow the same structure as described above, with the exception ofsignal field 720. This structure is an alternative that uses MACpartitioning to set the NAV of legacy stations. MAC partitioning field722 may contain frame information, coded at a legacy rate to allowreception by standard 11a and standard 11g stations.

Remaining set-up information 114 may include a plurality of supplementallong training sequences 724 and 726 and high data service field 728.Data field 708 may include a plurality of data symbols 730. Last datasymbol 732 may include tail and padding bits. One or more ofsupplemental long training sequences 720 may be expanded, as previouslydescribed.

Thus, within frame 710, legacy set-up info 712 and remaining set-up info714 may be joined to form the preamble of frame 710. The preamble maycorrespond to preamble 442 of FIG. 4. As discussed above, the preambleof frame 710 may be a Greenfield preamble if there is no valid standard11a or standard 11g preamble. For example, set-up info 506 of frame 504of FIG. 5 may be considered a Greenfield preamble. A Brownfield preamblemay have valid standard 11a or standard 11g preamble that informs legacydevices or components to stay off line, or off the network, so as to notinterfere or collide with frames using the standard 11n protocol. Frame710 of FIG. 7 may include a Brownfield preamble. Legacy set-upinformation 712 may include a 20 microsecond penalty when compared tothe Greenfield preamble. Both the Greenfield and the Brownfieldpreambles may be PHY headers.

Further, extra long training, such as long training sequence 718, may beused for MIMO channels, having multiple antennas. Because of themultiple antennas, more training information may be desired to bedelivered in frame 710 and the frames associated with transmit antennas1 and 2. Multiple sequences may be added in standard 11n to the preambleto indicate what frame 710 is. MAC partitioning field 722 alsofacilitates legacy set-up info 712 and indicates the particulars offrame 710 that is interpreted by legacy devices. Protection for frame710 may be provided at the MAC layer of a device, such as a legacydevice.

Frame 710 may be applicable to MIMO devices that use multiple antennasto exchange information or data within a channel. If the antennas wereto simultaneously transmit long training sequences, then a receiver maybe overwhelmed by the number of long training sequences received. Anoverwhelmed receiver may attempt to long train according to differentlong training sequences and treat different frames in an improper mannerby misreading legacy or remaining set-up information. Thus, for a numberof transmit antennas, a receiver may desire that number of pieces oftraining from each antennas. Thus, matrix, as described in FIG. 6, maybe generated but includes values for long training sequences withinapplicable frames. For example, supplemental long training sequences 724and 726 may be set up as a matrix that performs the long training for amultiple or group of antennas. This matrix may be a general matrix thatis sent over multiple antennas. In addition, applicable methods fordetermining the matrix also may be sent out over antennas. For example,there may be one long training sequence, such as long training sequence718, for each antenna. This method may take advantage of a long trainingsequence that has a low cross-correlation with itself. Further, thismethod may implement the shortest long training sequence but may invokean increased error. This method also may be more practical for a twotransmitter antenna, that includes values that alternate between evenand odd subspaces. Another method or process for determining the matrixis to transmit for multiple antennas may be applying a discrete Fouriertransform with a weighting matrix.

Another method for determining an applicable matrix may be transmittingzero everywhere such that only one transmitter is on at a time. Theoverall receive power may be less if a reduced probability of errorthrough. Another method of process may be a sub-null that uses the samesequence to create another way of orthoganality. For example, the firsttransmitter may transmit even subspaces and a second transmitter maytransmit odd subspaces too. A resulting matrix may include values thatalternate between

The preceding discussion has presented various embodiments for preamblegeneration for wireless communications in a wireless communicationsystem. As one of average skill in the art will appreciate, otherembodiments may be derived from the teachings of the present inventionwithout deviating from the scope of the claims or their equivalents.

1. A method for transmitting a frame for wireless communication, themethod comprising: determining at least one first training sequenceaccording to a number of antennas for transmitting a frame; generating apreamble for said frame, said preamble including a second trainingsequence and said at least one first training sequence; and transmittingsaid preamble with said frame using said number of antennas.
 2. Themethod of claim 1, wherein said generating includes determining a typeof said preamble, wherein said type corresponds to a protocol standard.3. The method of claim 1, wherein said generating includes generating alegacy set up information field.
 4. The method of claim 1, wherein saidgenerating includes generating a remaining set up information field. 5.The method of claim 1, wherein said determining includes deriving amatrix for said at least one first training sequence.
 6. The method ofclaim 5, wherein said deriving includes determining a size for saidmatrix, wherein said size corresponds to said number of antennas.
 7. Themethod of claim 1, further comprising determining whether a legacydevice is receiving said frame.
 8. A method for transmitting data in awireless system, the method comprising: defining a matrix for trainingof at least one antenna; determining at least one first trainingsequence according to said matrix and a standard for wirelesscommunication; and generating a preamble including said at least onefirst training sequence and a second training sequence, wherein saidpreamble is compatible with said standard.
 9. The method of claim 8,further comprising transmitting said matrix.
 10. The method of claim 8,wherein said generating comprises inserting a signal field into saidpreamble.
 11. The method of claim 8, wherein said generating comprisesgenerating a legacy set up information field in said preamble.
 12. Themethod of claim 8, wherein said defining comprises defining said matrixaccording to a multiple input, multiple output configuration.
 13. Themethod of claim 8, wherein said at least one standard comprises acurrent wireless communication standard.
 14. The method of claim 8,wherein said at least one standard comprises a legacy wirelesscommunication standard.
 15. A device for wireless communication, thedevice comprising: a transceiver to transmit a signal, wherein saidsignal includes a frame having a preamble; a plurality of subcarriers totransmit said signal within a channel; and processing means to produceat least one data stream for said frame and to generate said preamble,wherein said preamble includes an expanded at least one first trainingsequence and a second training sequence.
 16. The device of claim 15,further comprising an inverse fast Fourier transform module for saidsubcarriers.
 17. The device of claim 15, wherein said processing meansincludes a processor.
 18. The device of claim 15, wherein saidprocessing means defines a matrix for said expanded at least one firsttraining sequence.
 19. A method for generating a frame for wirelesscommunications, the method comprising: generating a preamble having aplurality of first training sequences and a second training sequence,wherein said preamble is defined by a wireless communication standard;determining said plurality of first training sequences according to saidwireless communication standard; defining a channel according to saidwireless communication standard; and stimulating a set of subcarriersfor said channel with said plurality of first training sequences. 20.The method of claim 19, further comprising defining a matrix for saidplurality of first training sequences.
 21. The method of claim 19,wherein said generating said preamble comprises generating a signalfield within said preamble.
 22. The method of claim 19, wherein saidgenerating comprises generating a legacy set up information fieldincluding said second training sequence.
 23. The method of claim 19,wherein said generating comprises generating a remaining set upinformation field including said plurality of training sequences.
 24. Asystem for transmitting a frame for wireless communications, the systemcomprising: determining means for determining at least one firsttraining sequence according to a number of antennas for transmitting aframe; generating means for generating a preamble for said frame, saidpreamble including a second training sequence and said at least onefirst training sequence; and transmitting means for transmitting saidpreamble with said frame using said number of antennas.
 25. A system fortransmitting data in a wireless system, the system comprising: definingmeans for defining a matrix for training at least one antenna;determining means for determining at least one first training sequenceaccording to said matrix and a standard for wireless communication; andgenerating means for generating a preamble including said at least onefirst training sequence and a second training sequence, wherein saidpreamble is compatible with said standard.
 26. A system for generating aframe for wireless communications, the system comprising: generatingmeans for generating a preamble having a plurality of first trainingsequences and a second training sequence, wherein said preamble isdefined by a wireless communication standard; determining means fordetermining said plurality of first training sequences according to saidwireless communication standard; defining means for defining a channelaccording to said wireless communication standard; and stimulating meansfor stimulating a set of subcarriers for said channel with saidplurality of first training sequences.